1. Field of the Invention
The present invention relates generally to a fiber-reinforced, rapid-hardening, high early strength engineered cementitious composite (HES-ECC) concrete material. The resulting high early strength ECC materials exhibit high compressive strength within 4 hours while also exhibiting high tensile ductility for long-term durability.
2. Background Art
There is an increasing demand for durable high early strength or rapid-hardening concrete materials in repair and retrofit practices, particularly on roads where minimum traffic disruption is preferred. For instance, highway transportation authorities often require a pavement repair to be completed in 6 to 8 hours at night so that the lane can be opened to traffic the next morning. In the past two decades, intensive experimental investigations carried out by both academic and industrial groups have led to successful formulation of concrete mixtures that can attain sufficient compressive and flexural strengths at very early ages. With various early strength gain rates, these concrete mixtures obtain high early strength by using either proprietary rapid hardening cements or portland cement together with chemical accelerator admixtures.
Unfortunately, traditional concrete repairs often lack durability. It has been estimated that up to half of all concrete repairs fail. About ¾ of the failures are attributed to the lack of durability, with the remaining attributed to structural failures. Premature deterioration is more common in repair sites using high early strength concrete because many proprietary binder systems often perform unpredictably under various construction conditions. For example, reduced freezing-and-thawing resistance was found in some very high early strength concrete mixtures. Meanwhile, early age cracking, associated with autogenous shrinkage and/or thermal gradient under high temperature caused by rapid hydration, also exacerbates the deterioration. The lack of durability in concrete repair is fundamentally related to the brittleness, or lack of ductility in other words, in most repair mortars.
Conventional ductile Engineered Cementitious Composites (ECC) mixtures use Type I ordinary portland cement (OPC), which shows relatively slow strength development. As a high strength gain rate is desired, an alternative binder system is needed. In addition to strength gain rate, the selection of a binder system has to take into consideration material cost, workability, practice restrictions, and long-term durability. Furthermore, the binder must not interrupt the micromechanical conditions for multiple microcracking and tensile ductility in ECC.
Current state-of-the-art high early strength cementitious materials for rapid repair, including various rapid hardening cement-based mortars and polymer mortars, are all quasibrittle in nature. The incorporation of short reinforcing fibers, most commonly steel, glass and polypropylene fibers, without regard to proper design of fiber, matrix and interface, typically leads to a composite with tension-softening behavior and low-strain capacity despite improvement in fracture energy. Thus, these fiber-reinforced materials are not strain-hardening, and cannot be described as ductile.
The recent use of crack initiation and propagation control in composites favors long-term ductility. The effectiveness of this micromechanics design approach is highlighted through matrix microstructure tailoring described in Li et al., U.S. Pat. No. 6,969,423. However, the ECC compositions disclosed therein cannot offer high early strength. Moreover, the chemistry and cure of high early strength compositions are sufficiently different from ordinary cement such that crack initiation and propagation are expected not to be the same. The interaction of the rapid curing matrix with reinforcing fibers and crack initiators will be significantly different.